Hi,
open this CSS file:
/templates/YOUR_TEMPLATE/html/com_virtuemart/assets/css/flexibleVM2Global.css
scroll to the bottom and include this CSS rule:
@media (max-width: 767px){
.FlexibleCSS3Button .FlexibleCSS3ButtonQuantity {height: 31px;margin-top: -20px;padding-top: 20px;}
}
The quantity box will be visible by default on smartphones. You can increase the 767px to cover the bigger device resolutions.
regards.